Ergodox Infinity: Add EE_HANDS support. (#9527)

Including a guide to set these EEPROM values (in the readme).

keyboards/ergodox_infinity/readme.md

@@ -29,6 +29,9 @@ Input Club Infinity Ergodox](https://github.com/fredizzimo/infinity_ergodox/blob
 The Infinity is two completely independent keyboards, that can connect together.
 You have a few options in how you flash the firmware:
 
+- Add `#define EE_HANDS` to your config.h, initialize the EEPROM values (see below),
+  and then flash the same firmware to both halves.
+
 - Flash the left half, rebuild the firmware with "MASTER=right" and then flash
   the right half.  This allows you to plug in either half directly to the
   computer and is what the above instructions do.
@@ -43,6 +46,35 @@ You have a few options in how you flash the firmware:
   directly connect the right half to the computer.
 
 - For minor changes such as changing only the keymap without having updated
-  any part of the firmware code itself, you can program only the MASTER half.
+  any part of the firmware code itself, you can program only the MASTER half,
+  but it is safest to program both halves.
+
+### EE_HANDS initialization
+
+To initialize the EEPROM values for `EE_HANDS` to work properly, these steps should work.
+They only need to be done once, unless you reset the EEPROM later.
+
+  - Plug in the left keyboard half to the computer, and press its program button.
+
+  - Flash the left half with `make ergodox_infinity:default:dfu-util-split-left`
+    (If you need to use a different method to flash your keyboard, still run this command,
+    and abort it with Ctrl+C when the flashing attempts starts to print errors,
+    then flash the built firmware).
+
+  - On the left half, press the top vertical 1.5U key (second from the top in the rightmost column) once,
+    then the 1U key at the bottom in the opposite corner (bottom left corner).
+
+  - Plug in the right keyboard half to the computer, and press its program button.
+
+  - Flash the right half with `make ergodox_infinity:default:dfu-util-split-right`
+
+  - On the right half, press the top vertical 1.5U key (second from the top in the leftmost column) once,
+    then the 1U key at the bottom in the opposite corner (bottom right corner).
+
+  - Add `#define EE_HANDS` to the config.h file of your keymap, and build your firmware using
+    `make ergodox_infinity:keymapname`.
+
+  - After this, you can flash both halves with the same firmware, _without_ having to rebuild with
+    "MASTER=right" or risking a mirrored keyboard when connected the wrong way.
+    If you reset your EEPROM later, you'll have to follow these steps again, though.
 
-- It is safest to program both halves though.